The first of the relevant physiological discoveries, that of the distinction between sensory and motor nerves, is credited to Charles Bell (1774-1842). Bell [see figure 33] was born in Edinburgh and educated informally. Although he attended lectures at the University of Edinburgh, most of Bell's anatomical and surgical instruction was received from his older brother John, a noted physician. By the time Bell was in his twenties, he was already a well-respected surgeon and by 1799 he had been admitted to the Royal College of Surgeons in Edinburgh. In 1806, he moved to London and five years later became affiliated with the Hunterian School of Anatomy. It was in that same year, 1811, that Bell printed one hundred copies of his 36 page Idea of a New Anatomy of the Brain  for private circulation among his friends and colleagues.
In the New Anatomy, Bell employed anatomical evidence to support the assertion that the ventral roots of the spinal cord contain only motor and the dorsal roots only sensory fibers. In so doing, he overturned centuries of tradition in which it was implicitly assumed that nerve fibers were indiscriminate with respect to sensory or motor function and established the fundamental distinction between these two types of nervous processes. When, as we have already seen, this distinction was combined with a parallel sensory-motor associationism, it led in the hands of Bain and Spencer to the first properly psychophysiological psychology and, through Jackson and Ferrier, to the establishment of the sensory-motor paradigm as the basis of functional localization in the cortex.
The first of the relevant philosophical advances was provided by Thomas Brown (1778-1820). Brown was born at Kirkmabreck, Scotland and educated in philosophy and medicine at the University of Edinburgh where he took courses with Dugald Stewart, a disciple of Reid. In 1810, he was appointed to share the professorship of moral philosophy with Stewart and within a short time he had become renowned for the brilliance of his lectures. In 1820, after his premature death, these lectures were published in four volumes as Lectures on the Philosophy of the Human Mind . Their impact was immediate, undoubtedly because Brown managed to unite elements of two disparate traditions, the Scottish intuitionalism of Reid and the empiricism of Condillac. In so doing, he helped redirect both traditions.
Among a number of novel contributions, including an important critique of introspection based on Brown's belief in the absurdity of the idea that one and the same indivisible mind could be both the subject and the object of the same observation, Brown made two conceptual advances of fundamental importance in the history of experimental psychology. The first was to emphasize the "muscle sense." Before Bain, as we have earlier suggested, the associationists had neglected movement and action in favor of the analysis of sensation. Brown was the first philosopher in that tradition to move toward a more balanced sensory-motor view by including the sensory side of movement in his conceptualization of the problem of objective reference in perception.
Brown's second contribution involved his detailed elaboration of the secondary laws of association, which he termed "suggestion." Brown's formulation of these laws, which involved the relative duration, strength (liveliness), frequency, and recency of the original sensations as well as the reinforcement of one idea by others, provided later learning theorists with a basis for the attempt to explain not only the facts but the quantitative parameters of association.
During almost the same period, in Germany, another philosopher of mind, Johann Friedrich Herbart (1776-1841) was also concerning himself with quantitative relationships among ideas. Herbart [see figure 34] was born in Oldenburg and studied at the University of Jena under Johann Gottlieb Fichte, with whom he found himself in some disagreement. Provoked by Fichte's ideas, Herbart decided to work toward his own systematic philosophy and upon completion of study at Jena, went to Göttingen where he took the doctorate in 1802. There he remained until 1809 when he moved to Königsberg to assume the chair formerly occupied by Kant.
At Königsberg, Herbart began work on his psychology, publishing his Lehrbuch in 1816 and Psychologie als Wissenschaft  in 1824/1825. As is evident from this later title, Herbart believed that psychology could be both empirical (although he denied the possibility of experiment) and mathematical. Arguing that ideas ("presentations") are arrayed in time and vary in intensity, he attempted to create both a statics and a dynamics of mind and employed complex mathematical equations to describe an hypothesized system of principles of interaction among ideas.
Specifically, Herbart assumed that ideas of the same sort oppose one another while ideas of different sorts do not. Opposition progressively weakens the original idea in consciousness and, as a result, it eventually sinks below the threshold of awareness where it remains until the appearance of a similar idea in experience causes the original to rise at a speed proportional to the degree of similarity between the two ideas. Furthermore, as the original is pulled up by the new idea, similar ideas cling to it. Thus no idea can rise except to take its place in the unitary mass of ideas already present in consciousness. This is Herbart's famous concept of "apperception" in which an idea is not only made conscious but assimilated to the whole complex of conscious ideas, the apperceptive mass.
In these views, Herbart took several giant strides along the path that the new scientific psychology would eventually follow toward a complex, carefully worked out, quantitative recognition of the critical distinction between ideas above and below the threshold of consciousness. As the received history suggests, he was a transitional figure between Kant and Fechner; but in his rejection of the possibility of experimental verification and his inability to link his philosophy of mind to the physiology of the brain, he travelled only part of the way toward the "new" psychology. Before psychology could be taken into the laboratory, it needed methods; and the primary source of the early methods lay not in the philosophy of mind, but in the work of physiologists such as Purkyne and Weber, who made fundamental contributions to the experimental phenomenology of sensation, and Müller, who elaborated the doctrine of specific nerve energies that systematized the epistemological role of the nervous system as intermediary between the mind and the world.
Jan Evangelista Purkyne (1787-1869) was born in Libochovice, in Northern Bohemia and received his first formal education at a Piarist monastery. After completing the novitiate, he spent a year in study at the Piarist Philosophical Institute. In 1807, under the influence of the writings of Fichte, he left the order and traveled to Prague. Two years of work at the University of Prague and an additional three years as a private tutor preceded his decision to return to the university to study medicine. In 1819, at the completion of his medical studies, he published his doctoral dissertation, Beiträge zur Kenntnis des Sehens in subjectiver Hinsicht. This led in 1823 to his appointment as Professor of Physiology at the University of Breslau. In that same year, he reprinted his dissertation as the first volume of Beobachtungen und Versuche zur Physiologie der Sinne . The second volume, which followed in 1825, was sub-titled Neue Beiträge zur Kenntnis des Sehens in subjectiver Hinsicht.
The two volumes of the Beobachtungen are among the great intellectual achievements of the period and constitute a major point of transition in the emergence of experimental psychology. With extraordinarily acute ability to observe phenomenological detail, Purkyne explored the psychological consequences in visual experience of a series of experimental manipulations of the conditions of stimulation, including application to the eyeball of pressure and electrical current, alteration in point of light exposure relative to the fovea, degree of eye movement, and variation in the intensity of light. While Purkyne is best known to psychologists for his classic descriptions of phenomena such as the change in apparent luminosity of colors in dim as opposed to bright daylight (the so-called "Purkyne effect"), it was the breadth and systematicity of his use of the experimental method to explore the parameters of sensory experience that helped lay the foundation for future laboratory work.
Ernst Heinrich Weber (1795-1878) was born in Wittenberg and educated at Leipzig, where he remained to serve as Professor of Anatomy from 1818 and of Physiology after 1840. In 1834, he published De pulsu, resorptione, auditu et tactu [37, see figure 35]. In that portion of the work devoted to touch, Weber presented an extensive experimental exploration of the sensory phenomenology of tactile experience. Whereas Purkyne had shown the value of applying the experimental method to the phenomenology of sensation, Weber extended the approach beyond experimentation to quantification.
Coining the phrase, just noticeable difference (JND) to refer to the smallest perceptible difference between two sensations, Weber amassed data in support of the general principle that a JND in the intensity of a sensation is a function of the change in the magnitude of a stimulus by a constant factor of its original magnitude (ÆR/R). Although it has since been shown that there are significant limitations in the generality of this relationship not only across other sensory systems but even within touch itself, it would be hard to overestimate the importance of Weber's discovery for the emerging science of psychology. In articulating the relationship which Fechner later termed "Weber's Law," Weber provided an existence proof for the possibility of establishing quantitative relationships between variations in physical and mental events. By linking these relationships to the nervous system, he helped, with Müller, to establish the epistemological function of the nervous system in mediating the relationship between mind and the physical environment.
Johannes Müller (1801-1858) [see figure 36] was born in Coblenz and educated at the University of Bonn. He received his medical degree in 1822 and, after a year in Berlin, was habilitated as privatdozent at Bonn, where he rose eventually to the professoriate. In 1833, he left Bonn to assume the prestigious Chair of Anatomy and Physiology at the University of Berlin. His most important contributions to the history of experimental psychology were the personal influence that he exerted upon younger colleagues and students, including Hermann von Helmholtz, Ernst Brücke, Carl Ludwig, and Emil DuBois-Reymond, and the systematic form he gave to the doctrine of the specific energies of nerves in the Handbuch der Physiologie des Menschen für Vorlesungen , published between 1834 and 1840.
Although Müller had enunciated the doctrine of specific nerve energies as early as 1826, his presentation in the Handbuch was more extensive and systematic. Fundamentally, the doctrine involved two cardinal principles. The first of these principles was that the mind is directly aware not of objects in the physical world but of states of the nervous system. The nervous system, in other words, serves as an intermediary between the world and the mind and thus imposes its own nature on mental processes. The second was that the qualities of the sensory nerves of which the mind receives knowledge in sensation are specific to the various senses, the nerve of vision being normally as insensible to sound as the nerve of audition is to light.
As Boring (1950) pointed out, there was nothing in this view that was completely original with Müller. Not only was much of the doctrine contained in the work of Charles Bell, the first of Müller's two most fundamental principles was implicit in Locke's idea of "secondary qualities" and the second incorporated an idea concerning the senses that had long been generally accepted. What was important in Müller was his systematization of these principles in a handbook of physiology that served a generation of students as the standard reference on the subject and the legitimacy he lent the overall doctrine through the weight of his personal prestige.
After Müller, the two problems of mind and body, the relationship of mind to brain and nervous system and the relationship of mind to world were inextricably linked. Although Müller did not himself explore the implications of his doctrine for the possibility that the ultimate correlates of sensory qualities might lie in specialized centers of the cerebral cortex or develop a sensory psychophysics, his principle of specificity lay the groundwork for the eventual localization of cortical function and his view of the epistemological function of the nervous system helped define the context within which techniques for the quantitative measurement of the mind/world relationship emerged in Fechner's psychophysics.
Wozniak, Robert H. "Mind and Body: Rene Déscartes to William James"
Bryn Mawr College, Serendip 1995
Originally published in 1992 at Bethesda, MD & Washington, DC by the National Library of Medicine and the American Psychological Association.
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